The present invention relates to catalyst supports of activated carbon, and more particularly catalyst supports comprising a substrate of activated carbon fibers.
In the chemical industry, it is common practice to use catalysts deposited in a finely divided state on substrates.
The substrates used are frequently activated carbons in the form of grains, powders, or extrusions. Their use poses a certain number of problems: micropores that are sometimes difficult of access; the need to sort grain sizes by screening; filtering to eliminate fines; and the risk of grains that are piled up in a reactor becoming crushed; which lead to precursors being selected that give rise to hard carbons (coconuts, olive-stones), but that do not always give good porosity, and that have the potential to create paths for liquids through a bed of carbon grains which leads to the available surfaces not being used to the full.
Proposals have been made for catalyst supports that do not rely on grains, but on carbon fibers of high specific area.
Fibers of small diameter offer a larger exchange area per unit mass of support. In addition, they can be disposed in the form of organized fabrics.
Thus, document FR 2 521 873 A discloses the use of a felt of active carbon or of activated carbon on which a metal catalyst can be deposited such as ruthenium or an association of ruthenium and palladium, and the resulting catalyst support is intended for use in catalytic conversion of glucose into sorbitol. Reference could also be made to document EP 0 299 637 A1 which discloses a support for a metal catalyst, the support being made of a woven cloth of carbon fibers having high specific area, about 1200 square meters per gram (m2/g). Before the catalyst is fixed, the cloth is washed in an acid solution and then rinsed. Such treatments carry a penalty in terms of time and cost of preparing catalyst supports, and the same applies to the oxidation treatments which are frequently performed on activated carbon substrates to augment the density of active sites per unit area prior to fixing the catalyst.
An object of the invention is to provide a simple method of preparing catalyst supports comprising a substrate of activated carbon fibers and particularly suited for use in fine chemical catalytic reactions that require a high degree of selectivity.
According to the invention, a method of preparing a catalyst support comprising making a carbon fiber fabric having high specific surface area and fixing a catalyst on the fabric is characterized by the fact that a rayon-precursor carbon fiber fabric is used.
In a first implementation of the method, the making of the carbon fiber fabric having high specific area comprises:
carbonizing a rayon fabric which, during a final stage, is-made at a temperature lying in the range 1000xc2x0 C. to 1300xc2x0 C., for a duration lying in the range 0.7 minutes (min) to 1.3 min; and
the carbonized rayon texture is activated.
Advantageously, the carbonization of the rayon fabric includes the stage of precarbonization performed at a temperature lying in the range 350xc2x0 C. to 420xc2x0 C., and the activation is performed at a temperature,lying in the range 850xc2x0 C. to 950xc2x0 C., e.g. under an atmosphere of carbon dioxide.
In a second implementation of the method, the making of the carbon fiber fabric having high specific surface area comprises:
impregnating a rayon fabric with a composition containing at least one inorganic ingredient having a function for promoting dehydration of the rayon; and
performing heat treatment at a temperature lying in the range 350xc2x0 C. to 500xc2x0 C. The impregnating composition can contain an ingredient selected from phosphoric acid, zinc chloride, potassium sulfate, potassium hydroxide, diammonium phosphate, and ammonium chloride.
Advantageously, the heat treatment comprises raising the temperature at a rate lying in the range 1xc2x0 C./min to 15xc2x0 C./min followed by a pause at a temperature lying in the range 350xc2x0 C. to 500xc2x0 C.
Both the first and the second implementations of the method make it possible to obtain a carbon fiber fabric having a specific area greater than 800 m2/g, and even greater than 1200 m2/g.
In addition to the possibility of obtaining large specific surface area, the use of rayon-precursor carbon fibers provides other particularly significant advantages. Thus, such fibers present a microstructure that favors the formation of surface functions or xe2x80x9cactive sitesxe2x80x9d and consequently favors catalyst fixing without necessarily requiring special surface oxidation as is usually the: case with grains of active carbon.
In addition, rayon-precursor carbon fibers, once activated, can present pores with a mean size lying in the range 0.3 nanometers (nm) to 3 nm for filaments with a diameter lying in the range about 5 micrometers (xcexcm) to about 20 xcexcm, and with a total porosity of 30% to 50% by volume. This favors great dispersion of the catalyst in the form of fine particles of a size not exceeding 3 nm, thereby obtaining high efficiency during catalytic reactions.
Another advantage consists in the high purity of the resulting carbon fibers: a carbon content greater than 99%, an ash content less than 0.3%, and an alkaline impurity content of less than 1500 parts per million (ppm). Thus, acid washing treatment prior to catalyst fixing is not necessary. In-addition, fibers make it possible to form substrates that are particularly suitable for receiving metal catalysts such as, in particular, ruthenium, platinum, rhenium, palladium, iridium, nickel, . . . or a combination of such metals, for uses in the field of fine chemistry, the purity of the carbon support being favorable to conferring the necessary selectivity. Furthermore, carbon derived from a rayon precursor is hydrophilic and consequently favors exchange with liquids, in particular aqueous media.
When the catalyst support is used in an oxidizing medium at a temperature higher than the temperature at which the carbon begins to oxidize, it is advantageous to provide the activated carbon fibers with a coating for protecting them against oxidation, e.g. a skin of silicon carbide. This can be formed by reacting the carbon of the fibers with silicon and/or a silicon compound such as silica.
In yet another implementation of the invention, the making of the carbon fiber fabric having high specific surface area comprises:
carbonizing a rayon fabric;
impregnating the carbonized fabric with a composition containing a precursor of a material for protection against oxidation; and
heat treatment for transforming the precursor so as to form a coating for protection against oxidation on the surface of the carbon fibers, the resulting fabric having a specific surface area that can be greater than 10 m2/g and can be as much as a few tens of m2/g, enabling the catalysts used to be deposited in an oxidizing medium.
Various known methods of fixing the catalyst to the activated carbon fiber fabric can be used, such as cationic exchange or liquid impregnation. They are advantageously performed continuously by causing the fabric to travel through one or more baths.
The carbon fiber fabric is essentially a two-dimensional fabric which is shaped after the catalyst has been fixed so as to ensure that the catalyst is distributed in substantially uniform manner throughout the support. This shaping is performed so as to make a support that matches the volume of a reactor to be occupied. For example, it can comprise operations of rolling, winding, or indeed needling plies so as to impart cohesion to the shaped support.